Power distribution/generation system

ABSTRACT

A power distribution/generation system is disclosed for supplying electrical power to a number of sites ( 32, 33, 34 ), one or more of which has a generator ( 53, 1 ) such as a Stirling engine ( 1 ) which is capable of generating electrical power. The generators ( 53, 1 ) are linked together on a local network that is connectable to an external power grid ( 31 ). A controller ( 35 ) can hold the distribution of power so that a site is supplied with electrical power from the local network if its power demand exceeds the power generated by the generators in that network. However, if the total power demand of all the sites in the network exceeds the total power available from the generators in that network, then the controller ( 35 ) causes power to be drawn from the grid ( 31 ) instead.

FIELD OF THE INVENTION

The present invention is directed to the delivery of energy toconsumers, and more particularly to a system which integrates on-siteenergy generation capabilities with conventional centralized powerdistribution networks.

BACKGROUND OF THE INVENTION

Conventionally, the delivery of various types of energy to consumers,such as industries, commercial entities, and residential customers, hasbeen carried out by regulated agencies. For example, in the UnitedStates the distribution of electrical power has been serviced by a fewthousand regulated monopoly franchises. In many cases, all of the energycustomers within a given geographic area rely upon a single electricalpower distribution company for their entire supply.

From the standpoint of the customer, certain inconveniences areassociated with the concentration of power distribution in a singleentity. Foremost among these is the reliability with which the power isdelivered. The ability of a power company to deliver adequate amounts ofenergy to all of its customers is dependent upon a variety of factors.Among these factors, the one which has perhaps the most significantimpact is the weather. Catastrophic weather conditions, such ashurricanes, tornadoes, ice storms, and the like, can severely disruptthe power distribution facilities, causing customers to lose access topower for hours, days or even weeks at a time. Increasingly volatileweather patterns have exacerbated this problem. Power reliability isalso adversely impacted by construction and motor vehicle accidents thatdisrupt power lines.

Another factor, which is sometimes related to the weather, is usage. Forinstance, during hot summer months, the demands of air conditioning andrefrigeration systems may surpass the capacity of the power distributionsystem during peak periods. As a result, the amount of power deliveredto each customer is reduced, resulting in so-called “brown-out”conditions. Under these conditions, certain types of equipment may notoperate properly, or may fail to operate at all, due to voltage levelsthat are below minimum specifications, and/or fluctuations that arecreated by an electrical utility in balancing of loads. This problembecomes more acute with the increasing use of various types of low-powerdigital electronic equipment, such as computers, which are much moresensitive to variations in voltage levels. Frequent fluctuations inpower quality such as dips, surges, sags and spikes are a significantsource of annoyance and disruption to consumers. These and other powerquality inconsistencies are driven mainly by the factors describedabove: weather, accidents and grid congestion.

Another source of inconvenience associated with centralized powerdistribution is the unpredictability of costs. The cost to traditionalutilities of providing power to consumers changes with the season andtime of day, in large part due to scarcity of distribution capacity. Inan effort to persuade consumers to reduce their usage during peakperiods, energy companies may impose higher rates on power consumptionbased on time of day or power grid usage levels. As a result, consumer'sbills are significantly increased if they must use power during thesetimes, making it more difficult to predict monthly or yearly energycosts.

Finally, centralized power generation has deleterious environmentalimpacts. Key environmental concerns associated with power plants are airemissions, water use and aesthetic objections. The distribution andtransmission grid also poses both aesthetic and potential environmentalhazards. Government regulations to make power generation moreenvironmentally friendly, as well as on plant and grid construction haveimposed new cost pressures on power plants, thereby increasing the priceof the energy to the consumer.

In an effort to alleviate some of these inconveniences, particularlythose associated with the unreliability of power delivery, consumers mayinstall a local back-up system. Typically, this type of system maycomprise one or more electric power generators that operate on fuelssuch as natural or liquid gas. These generators are designed to replace,or supplement, the power delivered via a centralized electric power gridduring those times when the centralized power is not available, or isinsufficient to meet the consumer's needs.

While the use of local generators provides some relief when centralizedpower is not available, they do not offer a totally satisfactorysolution. For instance, the purchase of the generators, and all relatedequipment, can represent a significant up-front investment for theconsumer, which may take years to pay for itself. Furthermore, theconsumer is required to perform regular maintenance on the generationequipment, even though it may not be used for a considerable period oftime. In addition, the quality of the power delivered by localgenerators may be insufficient to meet the consumer's needs, and aretherefore limited to use in emergency conditions.

It is an objective of the present invention, therefore, to provideon-site power generation capabilities to consumers that can beintegrated with the power delivered via a computer-driven centralizednetwork, to thereby ensure the reliable availability of power at apredictable rate, while avoiding the inconveniences typically associatedwith consumer-owned generation equipment.

SUMMARY OF THE INVENTION

Pursuant to the foregoing objectives, the present invention comprises amethod and system in which one or more electric power generators arelocated at or near a consumer's premises, to provide power which isdedicated to the needs of that consumer. In one embodiment of theinvention, the power provided by the on-site generators complements thatwhich is delivered via a centralized power grid network. For example,the on-site generators can be normally configured to provide power tocritical components of the consumer, such as refrigeration equipment,and the power requirements of other equipment can be supplied by thepower grid. In the event that the power grid is disabled, or isotherwise unable to provide adequate power to the consumer, the on-sitegenerators can be switched to provide power to the other equipment inlieu of, or in addition to, the principally supported components. Ifnecessary, the power that is supplied to the critical equipment, such asrefrigeration, can be cycled on and off, to balance the load on thegenerators.

In a further embodiment of the invention, a central control facilityselectively actuates the on-site generator(s) to intelligently arbitragebetween the locally generated power and that which is provided via thegrid network, based on a variety of factors. For example, theinstantaneous cost of power supplied via the grid network is provided toa processor in the control facility, where it is compared against storedcosts of operating the on-site generators. These costs might include theprice of fuel required to run the generators, maintenance expenses,other types of service and installation expenses, and finance charges,if applicable. When all of these costs are less than that powercompany's charges for the power provided by the grid network, thecentral control system can selectively actuate the on-site generators,to partially or totally replace power delivered via the grid. Since thecosts for operating the generators are known in advance, to a largedegree, it becomes possible to guarantee the consumer a maximum pricefor its power needs.

In addition to price-based considerations, other factors can also beemployed in the decision whether to activate the on-site generators. Forexample, data relating to weather conditions and peak usage periods canbe employed to actuate the generators at times when the delivery ofpower via the grid is likely to be interrupted or unreliable. In somecases, the utility may be willing to buy back some of the power which itwould otherwise provide to the consumer during peak usage periods, whichcan influence the decision to employ on-site generation.

As another factor, historical data regarding the consumer's power usagecan be employed to predict times when the usage requirements are likelyto be high, and thereby actuate the generators to supplement or replacethe power provided from the grid.

The features of the invention, and the advantages provided thereby, areexplained in greater detail hereinafter with reference to exemplaryembodiments of the invention illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a general block diagram of a first embodiment of a powersupply arrangement in accordance with the present invention under normalsupply conditions; and

FIG. 1 b is a general block diagram of a first embodiment of a powersupply arrangement in accordance with the present invention when powervia the grid is interrupted or diminished;

FIG. 2 is a general block diagram of a second embodiment of a powersupply arrangement, having off-site control of on-site generationequipment:

FIG. 3 is a more detailed block diagram of the central control facility;and

FIG. 4 is a block diagram of an application of the second embodiment tothe management of energy supply for multiple consumers.

DETAILED DESCRIPTION

Generally speaking, the present invention is directed to an arrangementin which power generation equipment is located at the site of aconsumer, and provides electrical power that supplements and/or replacesthe power delivered by a centralized power distribution network, such asthose affiliated with regional power utilities. To facilitate anunderstanding of the invention, it will be described hereinafter withreference to its use in connection with the power requirements ofcommercial enterprises and light industry. It will be appreciated,however, that the practical implementations of the invention are notlimited to these particular applications. Rather, in view of thereliability and economic advantages offered by the invention, it can beused by all types of electrical power consumers.

A simplified overview of one implementation of the invention isillustrated in'the block diagram of FIG. 1 a. An electrical powerconsumer 10 may have a number of different types of electrically poweredequipment, which are represented as various loads. For example, if theconsumer is a grocery store, some of these loads might includecomputers, lighting, ventilation and refrigeration equipment. Thesedifferent loads may have different levels of priority, as far as theirpower requirements are concerned. For instance, loss of power to theventilation equipment may pose an inconvenience, but would not requirethe store to immediately close. The computers and lighting may berequired for operation, and so the store may have to close temporarilyif they lose power, but is otherwise unaffected. In contrast, therefrigeration equipment is highly dependent upon a supply of reliablepower. Interruption of power to this equipment for an appreciable lengthof time could result in significant losses to the business because ofthe highly perishable nature of the inventory.

In the example depicted in FIG. 1, therefore, the power requirements forthe less critical loads, such as the computers, lighting and ventilation(depicted as Loads 1, 2 and 3 in the figure), are normally supplied viaa power grid 12 through which the consumer obtains its electrical energyfrom a local utility, an energy cooperative, or the like. In contrast,the more critical energy requirements of the refrigeration equipment,Load 4, are supplied by an on-site generator 14. Thus, even if theelectrical power from the grid should diminish and/or be interrupted,due to weather, excessive loading, etc., the critical load will remainoperational.

In the event that the supply of electricity via the power grid isinterrupted or diminished, the on-site generator 14 can be employed toservice one or more of the other loads which would be adversely affectedby the interruption. For example, the consumer may specify that, ifthere is a power outage, the lights and computers must remain operable,whereas the ventilation is not as critical. To accommodate thissituation, the individual loads can be selectively toggled between thepower grid and the on-site generator by means of associated transferswitches 16 a-16 d. In the event of a power outage, therefore, thetransfer switches for the lights and the computers can be switched toconnect them to the power supplied by the generator 14, as depicted inFIG. 1 b. To prevent an interruption in the power that is supplied tothe load as the switch is made from the power grid to the on-sitegenerator, and thereby provide virtual synchronization of thelocally-generated power with the grid, the switches preferably includean ultra capacitor, or the like, which can store and providehigh-wattage power for the brief period of time while the switching istaking place.

The decision to switch additional loads to the on-site generator 14 canbe based solely upon the ability of the power grid 12 to providereliable, high-quality power to these additional loads. For instance, ina fairly simple mode of operation, the lower priority loads can alwaysremain connected to the power grid 12, except when there is a completepower outage. In this case, their respective transfer switches 16 areactuated to connect them to the on-side generator 14. The actuation ofthe switches 16 a-16 d can be carried out manually by someone at theconsumer site, or automatically in response to sensors 17 that detect aloss of power from the grid 12, or a decrease in current and/or voltagebelow a preset threshold. In another implementation of this embodiment,the actuation of the switches 16 might be carried out by an off-sitecontrol facility which is informed of areas that have lost power via thegrid, and toggles the switches to connect them to the on-sitegenerators.

To accommodate the demand for increased on-site power supply that isrepresented in the situation of FIG. 1 b, various implementations can beemployed. In one approach, multiple generators can be installed at theconsumer's site, to provide a capacity equal to or greater than thehighest expected peak demand for equipment that has been designated ascritical by the consumer. For example, the requirements of therefrigeration equipment might be adequately handled by two Generators.To provide increased capacity during power outages, two additionalgenerators can be installed, and remain normally idle when they are notneeded. Additional generators can be located at the consumer's site foradditional redundancy. When the switches 16 a-16 d are actuated toswitch any of the loads from the power grid 12 to the on-site generator14, such action can also cause the additional generators to beautomatically turned on, and connected to the load. Preferably, whenmultiple generators are present, an on-site controller is employed tosense the level and quality of the power from the grid, and actuate theswitches 16 and generators 14 accordingly. The controller also sensesthe demands of the various loads, and operates to distribute the loadsamong the generators. The controller can be a general purpose or specialpurpose computer, for example.

Alternatively, or in addition, the power that is supplied to therefrigeration load can be cycled on and off, to balance it among therequirements of the other loads. This approach is practical for loadssuch as refrigeration, which are capable of operating effectively whilethe power is being cycled, due to the operational “inertia” inherentlyassociated with them. More particularly, once the perishable items havebeen cooled to an appropriate temperature, it becomes feasible to divertthe power to other loads until such time as the temperature rises to alevel that requires further cooling.

The on-site generation equipment 14 can be one or more of the varioustypes of self-contained power supplies. One example of such a powersupply is a fuel cell which is capable of meeting the on-site generationneeds of a consumer. In one preferred embodiment of the invention, theon-site generation equipment comprises microturbine generators. To beeffective in meeting the on-site generation needs of a consumer,particularly in situations where total loss of power from the gridoccurs, the on-site generator should possess the followingcharacteristics:

-   -   (1) Unlimited ability to follow changing loads;    -   (2) Ability to start and operate with full functionality whether        in conjunction with or independent from an external power        source, such as the power grid;    -   (3) Ability to provide unaffected service despite large,        unpredictable inrush currents such as those associated with        starting motors at a commercial or light industrial locations;    -   (4) Environmental emissions performance sufficient to allow        full-time operation without violating environmental regulations.        An example of a microturbine generator which possesses these        features is described in commonly assigned, co-pending        application Ser. No. 09/034,259, the disclosure of which is        incorporated herein by reference.

Generally speaking, the embodiment of FIG. 1 can be considered to be a“reactive” arrangement for the management of energy supplies, in whichthe decision to switch between the power grid and the on-site generationequipment is carried out in reaction to the state of the power grid. Ina further embodiment of the invention, additional factors beyond thestate of the power grid can be employed in determining whether toconnect loads to the power grid or the on-site generation equipment. Ageneral overview of this embodiment is depicted in FIG. 2. In additionto being reactive to conditions at the site, the embodiment of FIG. 2 is“pro-active” in operation, in that the decision whether to employon-site generation facilities is also based on a larger, and somewhatpredictive, range of input parameters.

As in the embodiment of FIG. 1, the embodiment of FIG. 2 includes one ormore loads at the consumer's site 10, which can be selectively connectedbetween the power grid 12 and on-site generation equipment 14 by meansof transfer switches 16. These switches are actuated in response tocommands that are provided from a control facility 18, as well as inresponse to sensors 17 or an on-site controller, as describedpreviously. The control facility also sends commands to the on-sitegeneration equipment 14, to cause it to start up or shut down, asnecessary. Preferably, the control facility 18 is located at a siteremote from the consumer, from which it is able to manage the energysupply for a number of different consumers.

The commands which are issued by the control facility 18 to actuate theswitches 16 and activate the generator 14 are based upon various typesof data from different sources. FIG. 3 illustrates the control facility18 in greater detail. The facility includes a processor 20, which can bea general purpose or special purpose computer, for instance. Thisprocessor receives the data and generates commands to control theswitches and on-site generators. Some of this data is received in realtime from external sources, whereas other data is stored at the controlfacility, in one or more tables and/or databases.

One type of data that is employed by the processor 20 is the pricing ofthe power that is supplied by the power company 22, through the powergrid 12. With the deregulation of the power companies, time-of-daypricing becomes more prevalent, and it is feasible to evaluate the costsof grid versus on-site power on a continuing basis. The example of FIG.3 illustrates the situation in which the price data is provided in realtime by the power company. As an alternative, the data may be publishedon a day-ahead or hour-ahead basis. In this case, it can be downloadedon a timely basis, e.g. from a site on the Internet, and stored locallyat the central control facility. This data is compared against the coststhat are associated with operating the on-site generation equipment 14.These costs can include the charges for fuel that is consumed by theequipment, such as the prices for natural gas or any other type ofhydrocarbon fuel that might be employed to run the equipment. Theseprices might be stored in a table 24 that is updated on a regular basisfrom information provided by the suppliers of the fuel. Other costs thatmight be included in this process include those associated with theregular maintenance of the equipment, the costs for the installation andmarketing of the equipment, which might be amortized over its life,finance charges, and the like. These costs could be stored in anothertable 26 within the control center 18. The processor 20 compares theaggregate of these generator-related costs against the rate structurefor the power company. From this comparison, a determination is madewhether it is more economical to employ power from the grid 12 or to usethe on-site generation equipment 14.

In effect, therefore, the control center 18 functions to arbitragebetween the grid power and the locally generated power. This capabilityis facilitated by having on-site power generation which is capable ofbeing substituted for the power grid, such as that provided by themicroturbine generators described previously.

The decision to switch between the two different power sources can bemade on a relatively simple basis. Whenever the cost of power suppliedover the power grid 12 is less than the aggregate costs of operating theon-site generator, the switches 16 can connect the loads to the powergrid. When the cost of grid power exceeds that of on-site generation,the appropriate switches are activated to connect the loads to the localgenerators. To avoid frequent switching between the two sources, forinstance when the respective costs are fluctuating in narrow ranges thatare close to one another, it may be preferable to employ a form ofhysteresis, or a minimum difference, before switching from one source tothe other.

In addition to pricing types of considerations, other factors arepreferably employed by the control center to determine when to switchbetween grid power and locally generated power. Current weatherconditions, such as temperature and humidity, can be received on areal-time basis and evaluated against statistical data 28 stored in thecentral control facility to determine the likelihood that an outage willoccur in the power grid. This evaluation can also include geographicallyrelated factors, such as the altitude of a particular consumer's site.In the event that there is a reasonable probability that a power outagemight occur at a consumer's site, based upon the statistical data 28,the control center can switch the loads over to the on-site generationequipment as a pre-emptory move, rather than wait until an actual outageoccurs. In addition to interruptions due to adverse weather conditions,the statistical data 28 can be used to predict when loads may change,prices may change, or the reliability of the grid may vary, and switchbetween the power sources accordingly.

Another factor in the switching decision can be historical usage data ofthe consumer. For each consumer, therefore, the control facility canstore a profile in a database 30, which might include the usage data,geographical data, etc. The usage data might indicate patterns of peakdemand that can be anticipated to determine when additional power may beneeded. For instance, in the case of a restaurant, the usage data mayshow that, at 4:00 a.m. each day, the load increases significantly, asgrills and ovens are first turned on. This information can be used todetermine whether to start an additional generator at that time, toaccommodate the increased demand.

The data profile can also include information regarding the operatingparameters of the on-site generation equipment which provides the mostefficient and/or economic operation. For example, if each generatoroperates most efficiently above a certain level of output power, it maybe advisable to turn one or more generators off if they are allcurrently operating below that level.

Each of these various factors can be appropriately weighted relative toone another and combined in the processor 20 to produce a decisionwhether to maintain the connection to the current power source or switchto the alternative one. If a decision is made to switch to thealternative source, a command is sent to a controller interface 32identifying the particular switches to be activated. If an on-sitegenerator needs to be started, the command can also identify this fact.In response, the controller interface sends signals to the designatedswitches and generators to carry out the necessary actions. Thesesignals can be transmitted through any suitable medium, such as viatelephone, cable, dedicated lines, over the Internet using TCP/IPprotocol, satellite and other wireless transmissions, etc.

At the consumer's site, the signals from the central control facility 18are received at equipment 34 that is analogous to a “set-top box” usedfor cable and satellite communications. For instance, if the internet isused as the medium to transmit the control signals, each receiver 34 canhave its own IP address for receiving packets of control informationfrom the central site. Preferably, the receiver 34 is implemented withinan on-site controller that functions to dispatch the power requirementsamong multiple generators, as described previously. Upon receipt, thereceiver decodes each packet and sends a command to the appropriateswitch 16 to connect its associated load to the power grid or on-sitegenerator, as required. The receiver can also send commands to theindividual oil-site generators to start up or shut down, as necessary.

In a preferred implementation of the invention, the consumer-sitereceiver can also provide information upstream regarding the status ofthe switches 16 and the on-site generators. For instance, each generatorwhich is currently operating can provide information to the on-sitecontroller regarding its power output. The controller can provide thisinformation to the central control facility 18, to thereby indicate thepercentage of each generator's capacity that is being utilized. If thepercentage reaches a threshold level, the control facility can issue acommand to start another generator, to thereby provide sufficient extracapacity in the event that a power boost is needed, for example when acompressor motor starts up. Conversely, if multiple generators areoperating at a low percentages the control facility can send a commandto shut down one or more of the generators, to thereby reduce on-sitepower generation costs. Alternatively, the distribution of the powerrequirements among multiple generators can be carried out locally by theon-site controller.

The upstream transmission of data to the central control facility alsoprovides usage data that can be employed to update the consumer'sprofile in the database 30. In addition to providing informationregarding the utilization of the individual on-site Generators, it maybe desirable to obtain information about the total power utilization atthe consumer's site, whether that power is being supplied via the powergrid or the on-site generators. This data can be obtained by sensing thecurrent consumption at each load, for example, and uploading it to thecentral control facility on a regular basis. e.g. every five minutes,once an hour, etc.

The example of FIG. 2 illustrates a single consumer's site connected tothe control center. In a practical application of the invention, thecontrol center can be employed to monitor and control the on-sitegeneration equipment of multiple consumers in a geographic area, asdepicted in FIG. 4. Such a grouping of consumer sites under the controlof a central facility can form a local network of controlled sites,which might encompass a well-defined area such as a city block orneighborhood. Each of these local networks can, in turn, be subnetworkswithin a larger network of power-managed sites. Some of the in-putinformation, such as pricing data and weather data, can be used tocollectively control the on-site generation equipment at all of theconsumers sites within the network. Other input information, such ashistorical usage data and consumer demand information that is stored inthe database 30, can be employed to selectively control each consumer'ssite individually or within the local network. While the example of FIG.4 depicts each consumer's site as having three on-site generators,labeled A, B and C, it will be appreciated that the sites could havedifferent numbers of generators which are suitable to handle theparticular requirements of those sites, respectively.

In another aspect of the invention, the on-site generation equipment canbe used to complement the resources of the power company. For example,in periods of high demand, the power grid may not have the capacity todeliver a reliable level of power to all consumers. Rather than impose abrown-out condition under these circumstances, the power company 22 canpresent requests to the control center to keep the on-site generationequipment operating during these periods, and thereby reduce the load onthe power grid. With this approach, the on-site generation equipment notonly facilitates the on-going operation of the individual consumer towhich it is connected, but also works to the advantage of all otherconsumers of the power company, by alleviating the possibility of anextended brown-out condition.

From the foregoing, it can be seen that the present invention providesan arrangement which enables on-site power generation to be successfullyintegrated with centralized power delivery, in a manner which providesthe consumer with power at the most economical rate available, whilemeeting the consumer's service needs as to reliability, power qualityand environmental responsibility. Furthermore, by switching betweencentrally delivered power and locally generated power in an intelligentmanner based upon a variety of factors, the present invention operatesin both a reactive and a pro-active manner to ensure a reliable sourceof power to the consumer at all times. As a result, it becomes possibleto effectively arbitrage between the power grid and the on-sitegeneration, so that power can be delivered to the consumer at aguaranteed constant price.

It will be appreciated by those of ordinary skill in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresently disclosed embodiments are therefore considered in all respectsto be illustrative, and not restrictive. The scope of the invention isindicated by the appended claims rather than the foregoing description,and all changes that come within the meaning and range of equivalencethereof are intended to be embraced therein.

1. A power distribution/generation system for supplying electrical powerto a number of sites, at least some of the sites comprising a generator,at least some of which are Stirling engines capable of generatingelectrical power, the generators being linked together on a localnetwork, the local network being connectable to an external power grid,and a controller to control the distribution of power so that a site issupplied with electrical power from the local network if its demandexceeds the power generated by that site's generator, and so that poweris drawn from the grid if the total power demand of all of the sitesexceeds the power generated by all of the generators.
 2. A systemaccording to claim 1, wherein the Stirling engine is a linear freepiston Stirling engine.
 3. A system according to claim 1, wherein thecontroller is arranged to export excess power to the grid if the powergenerated exceeds the power demand of the local network.
 4. A systemaccording to claim 1, wherein all of the generators in the local networkare routed through a hub which is then connected to the grid.
 5. Asystem according to claim 1, further comprising means to detect theabsence of mains power, wherein the controller is arranged to operate inthe absence of mains power to supply electrical power to selectedelectricity consuming apparatus.
 6. A system according to claim 5,wherein the controller is arranged, upon detection of the absence ofmains power to selectively supply electrical power to certain designatedemergency sockets within a site.
 7. A system according to claim 6,further comprising means to detect excess power demand, and to trim thepeak voltage supplied to the selected sockets for a predetermined periodof time.
 8. A system according to claim 1, wherein the cables whichcarry the power to and from each site are also used as a carrier for thecommunication signals between the sites.
 9. A system according to claim1, further comprising a power store in communication with at least oneof those sites that has a generator, the power store being arranged toreceive and store a proportion of the power generated by at least someof the generators with which it communicates for later distribution backto sites within the local network.
 10. A system according to claim 9,wherein the controller is further configured to control the distributionof power so that a first site is supplied with electrical power fromother generators within the local network, and/or the power store withinthe local network, if the demand at the first site exceeds the powergenerated by the generator at that first site, so that power is drawnfrom the power store if the total power demand of all of the sitesexceeds the power generated by all of the generators, and so that poweris drawn from the grid if the total power demand of all of the sitesexceeds the power generated by all of the generators and that poweravailable from the power store.
 11. The system of claim 9, wherein thepower store is selected from the list comprising a battery, a flywheel,pumped storage and superconducting magnetic storage.
 12. In a powerdistribution/generation system for supplying electrical power to anumber of sites, at least some of the sites comprising a generator atleast some of which are Stirling engines capable of generatingelectrical power the generators being linked together on a localnetwork, the local network being connectable to an external power grid,and a controller to control the distribution of power, a methodcomprising the steps of monitoring the power generated by eachgenerator; monitoring the power demand at each site; and controlling thedistribution of power so that a site is supplied with electrical powerfrom the local network if its demand exceeds the power generated by thatsite's generator, and drawing power from the grid if the total powerdemand of all of the sites exceeds the power generated by all of thegenerators.
 13. The method of claim 12, further comprising receiving andstoring a proportion of the power generated by at least some of thegenerators; and subsequently distributing the stored power back to thesites within the local network in response to an increased demand forpower.
 14. A power distribution/generation system for supplyingelectrical power to a number of sites, at least some of the sitescomprising a generator, at least some of which are Stirling enginescapable of generating heat and electrical power, the heat generated byeach Stirling engine being supplied to its respective site only, thegenerators being linked together on a local network, the local networkbeing connectable to an external power grid, and a controller to controlthe distribution of power so that a site is supplied with electricalpower from the local network if its demand exceeds the power generatedby that site's generator, so that power is drawn from the grid if thetotal power demand of all of the sites exceeds the power generated byall of the generators, and so that the power outputs of the generatorson the network are adjusted to maintain the local network voltage withinpreset limits, save that the power output of a Stirling engine on thenetwork is not reduced, where to do so would result in a reduction inthe desired heat output from the Stirling engine at that site, below ademanded level there.